textbook geoactive 2 chapter 3

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NATURAL HAZARDS IN AUSTRALIA

Chain lightning indicates the power and ferocityof natural hazards in Sydney.

Australia faces a range of effects from the physical environment — some of which develop into serious hazards and disasters. Crops can be destroyed by drought, beaches savagely eroded by storms, towns drowned by floods, lives and forests lost through bushfires, and buildings destroyed by earthquakes. In this chapter, we increase our understanding of the physical environment, its hazards and its impact on our activities.

A student:5.1 identifies, gathers and evaluates geographical

information5.2 analyses, organises and synthesises

geographical information5.3 selects and uses appropriate written, oral and

graphic forms to communicate geographical information

5.4 selects and applies appropriate geographical tools

5.5 demonstrates a sense of place about Australian environments

5.6 explains the geographical processes that form and transform Australian environments

5.10 applies geographical knowledge, understanding and skills with knowledge of civics to demonstrate informed and active citizenship.

• Comparing satellite images (page 51)• Interpreting a newspaper article (page 55)• Interpreting a satellite image (page 61)• Interpreting a newspaper article (page 65)• Working with choropleth maps (page 67)

Chapter 3

CHAPTER 3: NATURAL HAZARDS IN AUSTRALIA

47

alluvium: the loose material brought down by a river and deposited in its bed, flood plain or delta

bushfire: fire burning out of control in the open; also called a wildfire

cold front: boundary between warm air and advancing cold air

drought: a period of below average precipitationearthquake: series of shock waves that are generated by

a disturbance in the Earth’s crustEl Niño event: the reversal (every few years) of the more

usual direction of winds and surface currents across the Pacific Ocean. This change causes drought in Australia and heavy rain in South America. (Normally Australia has the rain and South America has the dry conditions.)

epicentre: the point on the Earth’s surface directly above the focus of an earthquake

fire front: the edge of the fire that spreads at the fastest rate

firebrand: aerial burning fuel that blows ahead of the fire front

firestorm: an intense fire, which may generate strong convection currents and violent winds that cause long-range spotting and flame spirals

flash flooding: sudden and destructive rush of water usually downhill following heavy rains on higher land

flood: an unusual accumulation of water that overflows from rivers, lakes or the ocean onto land that is not normally covered by water

flood mitigation: measures aimed at preparing for floods and trying to reduce their effects, such as constructing artificial levees, strengthening bridges, raising road levels and enlarging drains

flood plain: part of a river valley covered by water during floods

fuel: any material that burnsheatwave: a short period (usually a few days) of well

above average temperaturesLa Niña event: a period of well above average rainfall in

eastern Australia, which often brings floodsnatural disaster: occurs as a result of a hazardous

natural event that dramatically affects a communitynatural hazard: a natural event or object that is a

potential source of harm to a communityRichter scale: used to measure the energy of earthquakessclerophyll: plants found in low rainfall areas; their

leathery leaves help reduce water lossSouthern Oscillation: a major air pressure shift between

the Asian and east Pacific regions. Its most commonly known extremes are those referred to as El Niño events.

stubble: the stumps or stalks left in the ground when a crop, such as wheat, is harvested

subsidy: direct financial aid given by a government to an individual or group to reduce the price of a good or service

tropical cyclone: severe weather event that occurs when warm, moist rising air begins to spiral upwards. Tropical cyclones are often accompanied by very strong winds, heavy rain and rough seas.

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There is a difference between natural hazardsand natural disasters. A hazard is an event orobject that is a potential source of harm to a com-munity. A disaster occurs as the result of ahazardous event that dramatically affects a com-munity. There are six broad types of hazards:1. Atmospheric — for example, cyclones, hail-

storms, blizzards or bushfires2. Hydrological — for example, flooding, wave

action or glaciers3. Geological — for example, earthquakes or

volcanoes4. Biological — for example, disease epidemics

and plagues 5. Technological — for example, accidents, explo-

sions or hazardous materials6. Human — for example, war, crowd stampede

and terrorism.The first four groupings are generally referred

to as natural hazards and the last two as humanhazards. It should be noted that some naturalhazards are influenced by the actions andlocations of people. For example, the severity ofbushfires depends not only on the amount of fuelavailable or the strength and humidity of thewind. Humans influence bushfires through care-lessness in the use of fire, inadequate preparationof their property in the event of a bushfire andpoor disaster planning.

Risk analysis is concerned with the chance of ahazardous event occurring and whether or not itcould result in a disaster. The map oppositeshows those areas of Australia that are vulner-able to the threat of natural disaster. It showsthat different types of hazards and disasters areprevalent in different parts of Australia. Forexample, cyclones located to the north of Aus-tralia are associated with the warm tropicalocean; bushfires to the south of Australia areassociated with dry vegetation. People who moveto an area are usually aware if it is located in anatural hazard zone. For example, though Cairnsis subject to cyclones, the people of that commu-nity have decided that the benefits of living andworking there outweigh the risk factor.

ASSESSING THE RISK

Few Australian communities are free of the risk ofnatural disasters, but some are more vulnerablethan others. Vulnerability varies according to:• the location of the community and the

hazardous event• the magnitude of the hazardous event• the potential amount of damage.When disasters occur in unpopulated areas, peopleare rarely killed or injured, and the damage toproperty is small. However, when hazardous eventstake place in populated areas, a disaster can occur.

Managing hazardsEffective management of hazards requires thatindividuals, communities and governments:• prepare for hazards — for example, by clearing

vegetation around homes in bushfire-prone areas • prevent hazards — for example, by avoiding

establishing settlements on flood plains • recognise and respond to hazards quickly so

they do not turn into disasters.Emergency management procedures involve

gathering information, pre-disaster planning,responding, recovery and reconstruction. A dis-aster involves the coordination of voluntaryorganisations (such as the State EmergencyService), charities (such as the Salvation Army),financial assistance and donations, and govern-ment support (such as the army).

Australia’s worst natural disasters• Cyclone: Cyclone Tracy, Darwin 1974 —

65 deaths, 10 800 buildings destroyed,$4180 million cost

• Hailstorm: Sydney 1999 — 1 death, 24 800buildings damaged, $2000 million cost

• Bushfire: Ash Wednesday, Victoria andSouth Australia 1983 — 75 deaths, 2500buildings destroyed, $950 million cost

• Earthquake: Newcastle 1989 — 13 deaths,50 000 buildings damaged, more than$4000 million cost

• Landslide: Thredbo, 1997 — 18 deaths, 2buildings destroyed, $40 million cost

UNDERSTANDING AND COPING WITH DISASTERS

3.1NATURAL HAZARDS AND

NATURAL DISASTERS


49

Canberra

Perth

Hobart

Melbourne

Cairns

Alice Springs

Adelaide

Tennant Creek

Sydney

Darwin

Brisbane

0 500 km250

1990

Aivu 1989

Steve 2000

Rona 1999

Winifred 1984

Kathy 1984

Thelma 1998

Orson 1989

Chloe 1995

Annette 1994Bobby 1995

Ian 1992

Lena 1983

Ash Wednesday 1983

Gippsland 1998Ash Wednesday 1983

Ash Wednesday 1983

Dandenongs 1997

Nyngan 1990

Dalby 1981

Gympie 1999

Brisbane 1974

Cairns 1999

Pilbara 1980

Vance1999

BarryKatherine1998

1996

Les1998

Canberra 2003

Teresa 1992

Blue Mountains 1994Newcastle 1989

Gunnedah 2000

Charleville 1990

Tracy 1974

Wangaratta 1993N

The Katherine and Daly rivers communities in the Northern Territory are in a natural hazard zone where flash flooding can occur with little warning. In January 1998 the hazard became a disaster after Cyclone Les dumped 435 mm of rain within a couple of days. The Katherine River rose to a record 20.3 metres and the impact on the community was devastating — three people died, property and businesses were destroyed, hospitals evacuated, schools and roads closed, food supplies ran out and water supplies were contaminated. A crocodile was even seen in the main street. Emergency assistance involved the police, Northern Territory Emergency Service, Tindall RAAF air base and charitable organisations.

1. What is the difference between a natural hazard and a natural disaster?

2. Explain how a bushfire can be both a natural and human hazard.

3. Observe the map of natural hazards and disasters.(a) What type of natural disasters occur most often in

Australia?(b) Describe the location of Australia’s cyclone

hazard zone.(c) Describe the areas of Australia that are subject to

disaster fires.(d) Give one example of a community that suffered a

bushfire disaster.(e) What type of hazards are communities around

Newcastle subject to?(f) What would be the likely impact of a large

earthquake occurring in the earthquake hazard zone of central Australia?

4. Why does the risk of experiencing a natural disaster depend on the geographical location of a community?

5. Answer the key geographical questions in relation to the Katherine flood disaster.(a) Where was the disaster?(b) What was the impact of the disaster on the

community?(c) Why do people risk living there?

6. Go to www.jaconline.com.au/geoactive/geoactive2 and click on the Managing Disasters weblink forthis chapter. Create a document or web page containing weblinks that explains how one type of disaster can be managed.

Worksheets3.1 Extreme weather report

Australia’s natural hazards and disasters

Floods

Severe storms

At least one recorded tornado

Cyclones

6 to 10

Major cyclone

Major flood

Potential flash flooding

11 to 16

Potential flash flooding (greater frequency)

Approximate number of coastal crossings since 1959

At least one recorded severe thunderstorm (non-tornadic)

At least one severe thunderstorm per year (on average)

At least one severe thunderstorm per year (on average) and at leastone recorded tornado

Bushfires

Earthquakes

Risk area

Major earthquake

Areas subject to forest, grass and scrub fires of moderate risk to people

Major bushfire

Areas subject to disaster fires

http://www.jaconline.com.au/geoactive/geoactive2

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A drought is a prolonged period of belowaverage rainfall. It is a period of continuous dryweather when there is not enough water forusers’ normal needs. Because people use water inso many different ways and in such differentquantities, there is no universal definition of adrought in terms of amounts of rainfall.

The term ‘drought’ should not be confused withlow rainfall. Sydney could experience a droughtand have more rainfall during the period thanAlice Springs, which could be experiencing aboveaverage rainfall. If low rainfall meant drought,then most of Australia would be in drought mostof the time. Because different parts of Australiaare affected by different weather systems, thereis little chance that all of Australia would be indrought at the same time.

The Australian Bureau of Meteorology has his-torical records to show what is normal rainfall foran area and the area’s risk of drought. Droughtsaffect all parts of Australia over a period of time.Intervals between severe droughts have variedfrom 4 to 38 years. Some droughts can be localised

WHAT IS A DROUGHT? while other parts of the country receive good rain.Others, such as the drought of 1982–83, can affectmore than half of the country. Droughts can beshort and intense, such as the drought that lastedfrom 1 April 1982 to 28 February 1983; or they canbe long lived, such as the drought from 1 March1991 to 31 December 1995.

When drought comes to Australia, agriculturesuffers first and most severely — and eventuallyeveryone feels the impact.

Economic impactsDrought affects farmers through a decline in, ora loss of, the production of crops and livestock.This in turn affects the level of economic activityin rural towns and even large cities. The droughtof 1963–68 affected large parts of the continentand was the longest drought ever in arid centralAustralia. The last two years of this drought sawa 40 per cent decrease in the wheat harvest, theloss of 20 million sheep and a decrease in farmincome of around $500 million.

THE IMPACT OF DROUGHT

3.2NATURAL HAZARDS:

DROUGHTS

Aerial photograph of Warragamba Dam, June 2004. The dam is the main source of Sydney’s water supply. As can be seen, the water level in the dam became alarmingly low as drought conditions persisted in Australiaduring 2004.


51

The drought of 1991–95, one of the most severedroughts of the twentieth century over north-eastern Australia, resulted in total economiclosses estimated in excess of $5 billion. Even bigcities such as Sydney and Melbourne are forced toimplement water restrictions when they areaffected by droughts. Sydney stores more waterthan any other city of comparable size in theworld because it is frequently affected by drought.

Social impactsThe greatest social impact is the loss of income.This loss affects not only the farmers, but alsocommunities. In rural towns, for example, jobsmay be lost and businesses may fail. People maybe forced to leave drought-affected areas insearch of other work. Many never return. Pro-longed drought and the heartbreak associatedwith it can result in the breakup of families andsevere depression in individuals.

Environmental impactsDroughts have a large impact on topsoil inAustralia. During drought conditions, millions oftonnes of topsoil are blown away. This loss takesmany years to replace naturally, if it is everreplaced. This loss of topsoil can result in largeareas that are far less productive. Many cropcultivation methods in Australia are not suitablein a country that is often affected by drought.Prolonged droughts are usually associated withthe outbreak of serious bushfires. These bush-fires can have severe environmental impacts,even though much natural vegetation can benefitfrom fires.

1. What is meant by the term ‘drought’?2. What is the difference between drought and low

rainfall?3. Explain why there is little chance that all of

Australia would be affected by drought at the same time.

4. How often might droughts be expected to occur in Australia?

5. What are the two main types of drought that occur in Australia in terms of duration?

6. Describe the economic impacts of two droughts in Australia.

7. Why does Sydney store more water than any other comparable size city in the world?

8. Explain how droughts can result in decreased soil fertility.

9. Explain how the loss of income from a drought can have social impacts.

10. Imagine you are a wheat farmer in New South Wales on a property that has been affected by drought for over 18 months.(a) Describe the impacts of this drought on your

life.(b) What steps could you take in the future to

reduce the impact of possible droughts?

Comparing satellite imagesSatellite images often use false colours to highlight particular features. These two satellite images show how drought is spreading in Australia. Red areas indicate healthy vegetation.1. About what proportion of Australia had healthy

vegetation in September 2000?2. Describe the location of the main areas with

healthy vegetation in September 2002.3. About what proportion of Australia was affected

by drought in September 2002?

TOOLBOX

Source: © 2002 CNES/SPOT Imaging Services/Analysis by Agrecon

September 2000

September 2002

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Australia experiences considerable variation in itsrainfall. If we could take a ‘typical’ ten-year periodwe would have about four years of above averagerainfall, three average years and three belowaverage years. These fluctuations in rainfall haveseveral causes, many of which are not fully under-stood. Probably the main cause of major rainfallfluctuations in Australia is the SouthernOscillation, which is a major air pressure shiftbetween the Asian and east Pacific regions. Thestrength and direction of the Southern Oscillationis measured by a simple index called the SouthernOscillation Index (SOI). The SOI is calculatedfrom the monthly or seasonal fluctuations in airpressure difference between Tahiti and Darwin.

When there are ‘typical’ pressure patternsprevailing, the SOI is close to zero. If the SOIbecomes strongly positive this means that thesea-level air pressure at Darwin is much lowerthan normal and a La Niña event occurs. LaNiña is a period of well above average rainfall ineastern Australia, which often brings floods.

SOUTHERN OSCILLATION During an El Niño event, the SOI is stronglynegative and the sea-level air pressure atDarwin is higher than at Tahiti. An average yearoccurs when the SOI is between −10 and +10. ALa Niña event occurs when the SOI is above +10;an El Niño event occurs when it is below −10.

Probably the main cause of drought in eastern Aus-tralia is El Niño — a warm ocean current in thePacific. At irregular intervals, it spreads furthersouth and the water in the central and easternPacific becomes much warmer, bringing heavy rain-fall and floods to arid parts of South America.

At the same time, normally warm water in theoceans to the east and north of Australia arereplaced by much cooler water as the warm waterspreads east. As well, the easterly trade windsthat normally blow across the Pacific Oceanbringing warm, moist air to Australia reversetheir direction. There is an accompanying reversalof air pressure across the Pacific, resulting instrong high pressure systems building up over

EL NIÑO

3.3CAUSES OF DROUGHT

IN AUSTRALIA

Weather conditions in (a) a typical year, and (b) an El Niño year

Cold deep waterEl Niño

Typical

Ocean Cold upwellingceases.

Warm surface currents reverse.

Australia

Australia

Trade winds reverse direction.

Atmosphere

Atmosphere

SouthAmerica

SouthAmerica

Ocean

Warm surface water

Strong surface currents

Upwelling of colddeep water

Trade winds blow towards Australia.

Warm rising aircauses thunderstorms

and floods.

Dry sinking aircauses droughts.

Dry sinking air causes droughts.

Warm rising air causes thunderstorms and floods.

(a)

(b)


53

0 7500 km2500 5000

N

Wet

Equator

Tropic of Capricorn

Tropic of Cancer

Dry Warm

most of Australia. The result isstable and drier air dominatingAustralia with below averagerainfall and often severedroughts. El Niño brings signifi-cant climatic change, not only toAustralia but to other parts ofthe world.

In recent years scientists havemade great advances in under-standing and forecasting ElNiño and Southern Oscillationevents. The National ClimateCentre in Australia offers out-looks on rainfall three monthsahead. These outlooks are proving to be of greatvalue to farmers and especially valuable for eco-logically sustainable development in rural areas.

The SOI, El Niño and La Niña (eastern and northern Australia)

• El Niño is a Spanish term that translatesas ‘the boy child’. Peruvian anchovyfishermen traditionally use the term —an obvious reference to the ‘ChristChild’ — because El Niño usuallyappears around Christmas.

• The study of corals from the Great Bar-rier Reef shows that El Niño first ‘turnedon’ between 4000 and 5000 years ago.

• The first settlement in Australia wasestablished in Sydney during the bone-dry years of 1787–93. The Tank Stream,the most reliable source of water, ran dry.

autumndry

conditionscommence

lowwinterrainfall

lowspringrainfall

monsoonlate andgenerally

poor

droughtoften

brokenby heavy

rains/floods

El Niño

La Niña

Month

J F M A M J J A S O N D J F M A M J

So

uthe

rn O

scill

atio

n In

dex

posi

tive

nega

tive

0

Typical pattern of eastern and northern Australian rainfall and the SOI during an El Niño and La Niña episode

1. What is the Southern Oscillation?2. How is the SOI calculated?3. What do the following SOIs indicate?

(a) Between +10 and −10(b) > +10(c) < −10

4. Observe the diagram ‘The SOI, El Niño and La Niña’.(a) Describe the seasonal rainfall conditions that

prevail over the period of the El Niño event.(b) What happens to the SOI when the drought

breaks?(c) What would be happening to the air pressure at

Darwin as the SOI rises?5. Observe the diagram comparing a typical year and

an El Niño year and study the text. Fill in the missing words from the list below.

During an El Niño event, the normally sea in the oceans to the and of Australia are replaced by much water. The air pressure at begins to fall relative to the air pressure at . The normal easterly trade winds change their direction. The result is and air and severe

.

6. Observe the map showing the areas affected byEl Niño. Describe the areas that become (a) wetter, (b) drier and (c) warmer.

7. Go to www.jaconline.com.au/geoactive/geoactive2 and click on the Drought weblinks for this chapter. What long-term weather conditions are indicated by the latest SOI graph? How does the Bureau of Meteorology contribute to drought management?

stableTahiti

moistdrier

coolerwarm

eastnorth

droughtsDarwin

Areas affected by El Niño


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Australia’s drought of March 1991 to December1995 was a long-lived, El Niño related drought.It was one of the longest of the twentieth centuryand one of the most destructive in terms ofdamage to the physical environment. Largeareas of topsoil were lost and there was somedamage to vegetation and wildlife.

As droughts occur frequently in Australia,most native plants and animals are usually wellequipped to deal with them. However, introducedcrops and animals can be severely affected,leading to crop failures, minimal planting of newcrops and the death of introduced stock, such assheep and cattle. Consequently, droughts havethe most impact on areas given over to croppingor intensive grazing. The drought of 1991–95 hada devastating impact on such areas.

As the map opposite shows, the drought had itsgreatest impact on north-eastern Australia, par-ticularly Queensland. Most of eastern Australiaexperienced below average rainfall for much of theperiod of the drought.

The cause of the drought was clearly El Niñorelated, as the graph of the SOI for the periodshows. For nearly all of the period, the SOI wasbelow zero and for many months it was in therange >−10.

Southern Oscillation Index, 1989–95

One of the most dramatic consequences ofsevere drought is the dust storm. When the soillacks moisture and dries out, plants and treeroots that normally hold the soil together witherand die. The dry soil particles on the surface areeasily lifted into the air by strong winds, and top-soil can be carried across huge distances.

20

10

0

–10

–20

–30

– 40

Year1989 1990 1991 1992 1993 1994

SOI5 month mean

1995

So

uthe

rn O

scill

atio

n

Farmers reduced their stock numbers anddecreased the amount of land under crops. Someleft drought-affected areas permanently. Farmersattempted to reduce the impact of drought byimproving their farming methods, including theuse of conservation techniques and fodder pro-duction systems. Community-based groups suchas farming organisations lobbied for financialassistance from governments and provided assis-tance for individual farmers. The CommonwealthGovernment provided $590 million in droughtrelief from 1992 to 1995. The government hasnow developed the National Drought Policy,which provides a range of subsidies and directfinancial assistance.

1. What were some of the main effects of the 1991–95 drought?

2. Observe the graph of the Southern Oscillation Index, 1989–1995.(a) What indications were there that a drought

would occur in 1991?(b) According to the SOI, which were the two worst

drought years?(c) Describe the movement in the five-month mean

for the SOI over the period shown on the graph.3. Observe the map. Describe the areas affected by

drought. Use the map scale to estimate the size of the area that experienced the lowest rainfall on record.

4. In small teams, investigate how Australians respond to droughts. Decide which one of the following groups your team will focus on:(a) individuals(b) community-based organisations(c) governments.Summarise your findings in a poster, PowerPoint display or report and present it to the class. After the presentations, explain how the response of two of these groups demonstrates active citizenship. To get started, go to www.jaconline.com.au/geoactive/geoactive2 and click on the Drought Response weblink for this chapter.

RESPONSE TO THE DROUGHT

3.4THE DROUGHT OF

1991–95



55

Dust storm approaching the town of Griffith, New South Wales. The dust was picked up by 90-kilometre winds from drought-affected farms. Dust storms carry away millions of tonnes of precious topsoil.

Areas of Australia most affectedby the drought of 1991–95

Dust disaster to cost hundredsof millions

by Asa Wahlquist and Sonya SandhamThe massive wind storm that has blown millions of tonnes of dustacross south-eastern Australia this week was a natural disaster thatcould cost grain growers hundreds of millions of dollars in lost pro-duction, a senior CSIRO scientist has warned.

‘The estimated lost production value runs to perhaps 10 per cent ofthe value of the crops,’ Dr Michael Raupach, an environmental scien-tist, said. Grain crops grown in south-eastern Australia were worthabout $2 billion last year.

A thick cloud of dust hung over Sydney yesterday morning asgale-force winds carried more than one million tonnes of topsoil overthe city. At dawn the cloud was 500 kilometres long, 300 kilometreswide and 1.3 kilometres deep but by 11 am it had dissipated to adepth of 2.5 kilometres.

A senior research officer with the Department of Conservation andLand Management, Mr John Leys, said between 1 million and 1.5 milliontonnes of topsoil blew over the city, enough to fill 42 000 semi-trailers.He estimated that the dust storm in South Australia would have filled600 000 semi-trailers, enough to form a line from Sydney to London.

Farmers have already had to delay crop planting because ofdry conditions and now fear a drought. ‘It’s a combination ofstrong winds, both north-westerly and westerlies, with dry con-ditions which make the surface layer of the soil friable and easyto lift off and the fact this has occurred just before sowing time,when a lot of farmers have their fields open and exposed andready to sow,’ Dr Raupach said. ‘The other factor that has aggra-vated the situation is that some farmers have been burning offstubble, because of the mouse plagues in north-westernVictoria. The combination has left us pretty vulnerable.’

Dr Rapauch said the dust lifted into the storm comprisedthe finer particles. ‘They preferentially contain the nutrients,especially nitrogen and phosphorus, and have a lot to dowith the water-holding capacity of the soil. That means thatwhen we lose a tonne of dust from the soil we are in factlosing nutrient from between two and three tonnes of soil;we are winnowing the soil of its nutrients.’

One tonne of dust carries nutrients worth $1.50. ‘So if wehave lost 20 million tonnes we have lost $30 million ofnutrients …’

Source: The Sydney Morning Herald, 27 May 1994

Interpreting a newspaper articleNewspapers often provide valuable information on contemporary geographical issues. Daily newspapers are usually up to date and accurate, and we can obtain additional information from them in the form of photographs, graphs, diagrams and maps. Read the article about the dust storms of 1994, which were an important geographical issue and a strong reminder of the destructive power of droughts on the physical environment. Then answer the following questions.1. What is the estimated cost to grain growers of

the wind storm? How was this figure calculated?2. Why was so much soil eroded during the storm?3. What part of the soil was most affected by the

storm?4. What is the estimated value of soil nutrients

lost?5. With the aid of the diagram and the text of the

article, describe the passage of the dust storm across south-eastern Australia.

TOOLBOX

HIGH

HIGH

BrisbaneMoree

NewcastleSydney

Melbourne

Adelaide

CobarBroken Hill

LOW

Winds from SA pick up 10 to 20 million tonnes of topsoil

1 to 1.5 million tonnes of topsoil blow over Sydney

Perth

Hobart

Melbourne

Cairns

Alice Springs

Darwin

Sydney

Brisbane

AdelaideCanberra

0 500 1000 km

Serious rainfall deficiency

Severe rainfall deficiency

Lowest rainfall on record

N

Tropic of Capricorn

GEOactive 2

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A bushfire is a fire burning out of control inthe open. Bushfires can burn using grass,scrub or forest (or a combination of these) forfuel. Unless quickly controlled, bushfirescan become large, spreading to affect forests,wildlife, crops, houses and other buildings,and human life. In Australia, some bushfireshave become major disasters.

Fires are not a recent occurrence in Australia.Since the last Ice Age, bushfires have influencedthe development of the Australian land. Fires arean essential element in some Australian ecosys-tems, which need the intense heat of bushfires torelease the seeds from plants and replenishgrowth. Australian Aborigines used fires to assistthem in their hunting activities. It is believedthat the fire activities of Aborigines contributedto the development of an open woodland eco-system in parts of south-eastern Australia.

Early European settlers used fire to assist inthe clearing of land for crops and as a means ofremoving stubble following cropping. However, inmore recent times, laws have been passedrestricting the lighting of fires for these purposes.This has led to a more dense vegetation in manyrural areas and a greater accumulation of leaf and

bark litter on the ground. The litterprovides a significant amount of fuelfor fires if they do start.

Bushfires are one of the most destruc-tive forces of nature. Firefighters risktheir lives each year to control andeventually extinguish them. Eventhough bushfires can occur naturally,mainly as a result of lightning strikesand spontaneous combustion, most arestarted by the activities of people. Thisincludes cigarettes and matches beingcarelessly discarded, electricity cables,sparks from machinery and tools, andburning off. Arson has been the causeof some of the worst bushfires.

BUSHFIRES

WHAT CAUSES BUSHFIRES?

Eucalypts and bushfires

There are two main types of bushfires:• Surface bushfires burn in grass, low shrubs and

ground litter. They can travel at high speed butare relatively easy to control.

• Crown bushfires occur when heat and flamesfrom a surface fire ignite the crowns of trees.Crown fires spread rapidly if there are strong,hot winds and very dry vegetation. Hugeamounts of flammable eucalyptus vapour,transpired from leaves, can create firebrandsthat engulf the tree crowns ahead of the firefront. This makes crown fires very dangerousand difficult to control.

3.5BUSHFIRES AS

NATURAL HAZARDS

Crowns of trees may overlap, encouraging the rapid spread of fire.

Trees contain highly inflammable oil in branches.

Leaf litter builds up, providing fuel for surface fires.

Fibrous bark burns readily.

Birth of a fire tornado. Tornadoes usually form in the updraughts of thunderstorms. Winds interacting with the rapidly rising heat generated by bushfires can produce a similar result.

1. The rising heat of the fire creates vortices.

2. The vortices tilt upward creating a fire whirl or tornado.

3. The rotation intensifies as super-heated air rises higher.

Lightning in the smoke columns forms in much the same way as lightning in storm clouds —

through the building up of electrical charges caused by friction with rising air and smoke.

Fire tornadoes can hurl flaming logs and burning debris over

considerable distances.

Fire tornadoes increase wind speeds by up to 10 times.

Rotation speeds can reach 300 kilometres per hour.

Spinning columns of air are most likely created when rapidly rising heated air combines with the

effects of winds encountering obstacles and topographic features.


57

Canberra

Perth

Hobart

Melbourne

AdelaideSydney

Darwin

Brisbane

0 500 km250

Wildfire riskSubject to fires; little risk to people Subject to forest, grass and scrub fires; moderate r

Subject to disaster fires

N

Tropic of Capricorn

As the map shows, bushfires usually occurin the less arid parts of Australia. Aridareas tend not to have enough fuel to sus-tain fires for any length of time. South-eastern Australia is particularly subject tobushfires. There are several reasons for this,including the following:1. Large areas are covered with sclerophyll veg-

etation. The dominant trees are eucalypts,which have highly flammable oil in theirbranches. Eucalypts burn readily and canbecome so hot during fires that their sap boilsand the whole tree or shrub can explode inflames. This is not a problem for the plantspecies as most eucalypt varieties burn hotand fast as a means of releasing seeds onto thefire-cleared ground. Some burnt trees simplysend up new shoots from their stumps or rootsand grow to full-size trees again.

2. The area is also subject to prolonged periods ofbelow average rainfall and droughts. Many ofthe most severe bushfires have occurredduring droughts and El Niño events. Bushfiresand drought often occur together.

3. South-eastern Australia is also subject toheatwaves and strong northerly winds duringthe summer months. Two or three days ofheatwave weather can often provide conditionssuitable for bushfires to occur.

Bushfires can bring massive destruction, loss oflife and personal hardship to families and com-munities. Individuals, community-based groupsand governments have responded to this hazardin many ways.

There are over 70 000 individuals who are vol-unteer members of bushfire brigades. They domost of the fighting of bushfires. During times ofsevere bushfires they can be on duty for days at atime, sometimes facing extreme danger. Theymake a highly significant contribution to makingour communities safer.

The government has set up many agenciesthat work to protect the community when bush-fires occur. These include fire brigades, police andambulance services, welfare agencies and theState Emergency Service. Governments can pro-vide emergency financial assistance in severebushfires. The Bureau of Meteorology providesshort- and long-term weather forecasts to warnof bushfire dangers.

WHERE DO BUSHFIRES OCCUR?

RESPONSE TO BUSHFIRES

Bushfire brigade volunteers risk their lives to save people, animals and property from bushfires.

1. What is a bushfire?2. How have bushfires been an essential input into

some Australian ecosystems?3. How did the activities of Aboriginal people affect

ecosystems?4. How have fire restriction laws affected vegetation

in rural areas?5. What is arson? How could arson be the cause of

some of the worst bushfires?6. How could very strong, hot winds affect crown

fires and make them very dangerous?7. Observe the map showing the risk of bushfires in

Australia. Describe the distribution of the risk areas. Why is south-eastern Australia particularly subject to bushfires?

8. Explain how a fire tornado could develop during a bushfire. How would a fire tornado affect the rapid spread of a bushfire?

9. Go to www.jaconline.com.au/geoactive/geoactive2 and click on the Bushfire weblink for this chapter. Explain the links between El Niño and bushfires in Eastern Australia.

10. Using the resources in your library and/or the Internet, write a brief report on the work of bushfire brigades.

Worksheets3.2 Survivor scavenger hunt

Risk of bushfiresin Australia


GEOactive 2

58

Bushfires are one of the most commonnatural disasters faced by Australians. Alongwith floods and droughts, they are part of theway our natural environment functions. Butbushfires can and do kill. In 1983, Australia’sworst bushfire disaster, known as AshWednesday, left 75 people dead as well as2500 buildings and 520 000 hectares of forestand farmland destroyed in South Australiaand Victoria.

1. What is a crown bushfire?2. List the fire-fighting techniques shown

here.3. Why do many animals die in

bushfires?4. Why do bushfires often occur in

times of drought?5. How do eucalypt trees help

bushfires spread?6. Imagine a small fire front with a

long flank. The fire is being pushed by winds from the north. Suddenly the wind changes and starts blowing from the west. Will the people on the west or on the east of the original fire now be in danger?

7. Write a news report of the scene shown on these pages. Outline the effects on people and wildlife. Include interviews and describe the fire using key terms explained in this chapter.

8. How could you make your home and community safer in a bushfire? Think about terrain, climate, vegetation, access to water and fire-fighting resources, and so on. Design a poster outlining one of your ideas. It should be eye-catching and contain a short, clever message. Use information in this spread to help you, and go to www.jaconline.com.au/geoactive/geoactive2 and click on the Fire Prevention weblinks for this chapter.

3.6FROM HAZARD TO

DISASTER

Dry conditions caused by drought, searing temperatures and strong, hot northerly winds cure the bush, making it so dry that a spark can ignite a major bushfire. Grasses die off and the soil is easily blown away.

High temperatures, low relative humidity, and strong winds combine to create high fire danger days.

Many animals perish, as fire fronts often move too quickly for them to escape.

Crown bushfires spread through the treetops or ‘crowns’ of forests. Before long, a wide blanket of forest is fully ablaze.



59

Australia’s eucalypt forests not only tolerate fire but also need it in order to survive! The seeds of some eucalypts need the heat of a bushfire to be able to open and grow. The low moisture content of eucalypts means they ignite and burn easily. Their fibrous bark is highly combustible.

Special helicopters can scoop up to 9500 litres of water in 45 seconds and dump the whole lot in just 3 seconds.

What was the flank or side of a bushfire can become the new fire front if there is a wind change.

Dry forests provide plenty of fuel. Surface bushfires quickly ignite dry, flammable grass, twigs and branches on the ground.

Properties are more likely to survive if gutters are clear of leaves, lawns and shrubs are trimmed, and there is access to water and hoses. People who defend their house must cover up with cotton or woollen clothing.

By using the wrong building materials, planting eucalypts close to the house and stacking firewood against the house, people can actively contribute to the spread of a bushfire.

A firebrand is burning fuel that is pushed ahead of the fire front by the wind. Firebrands have been known to travel kilometres from their original source. A spot fire is a new bushfire started by firebrands.

GEOactive 2

60

W a l

Fires

P A C I F I C

O C E A N

Sandstone rock dries out quickly.

War

mer north

erlyslo

pe

War

mer north

erlyslo

pe

ConvectioncurrentsConvectioncurrents

Air movements createdin gaps and valleys help forcethe fire uphill.

Spot fires started as burningdebris falls down the hillside.

Hot winds

North SouthWinds at the top of the hill carryfirebrands to other areas, starting spot fires.

Smoke clouds

From Saturday 1 January to Saturday 15 January1994, New South Wales experienced its mostdevastating bushfires on record. There had been aprolonged El Niño event — much of the coast ofNew South Wales had endured an extended periodof below average rainfall. This had resulted indrought conditions in many areas and a build-upof very dry fuel. There were repeated warningsthat the fire potential was the worst for more than20 years.

Fires flared and receded and entire townswere evacuated — some more than once. Morethan 200 fires raged over large areas of coastalNew South Wales, from Tweed Heads in thenorth to Ulladulla in the south, and many ofthese were started by arsonists. Thebushfires were particularly largeand dangerous around Sydney. TheKu-ring-gai and Lane Cove nationalparks were ablaze and severalhomes on Sydney’s north shore werelost.

The fires were eventually broughtunder control, but only after amassive firefighting effort. There wereover 10 000 firefighters involved,14 000 fire units and water tankers,51 helicopters and five fixed-wing,water-bombing aircraft.

There are some parts of Sydney andnearby areas where fires can spreadquickly due to the landforms. These are thesandstone plateau areas where sandstone rockoverlies shale rock. In dry periods the vegetationon the plateau dries out rapidly making it idealfuel if a bushfire starts in the valleys below. Thenorthward-facing slope and upper valley alsotend to dry out rapidly as they are under theinfluence of northerly winds and more directsolar radiation (see the diagram). Fires canspread very quickly and engulf the plateau area,placing houses at severe risk. Many nationalparks around Sydney are also very vulnerablebecause they have large areas of these land-forms, as are suburbs such as Terrey Hills andLane Cove.

DANGER ZONES

Distribution of the New South Wales fires, 1994

Sandstone plateaus and the spread of bushfires

We will examine one of the 15 days, 7 January1994 — a day described by the Police MinisterTerry Griffiths as ‘catastrophic’ — and analysethe main fire events.

The weather situation was typical of thoseassociated with bushfires. The synoptic chart for7 January 1994 shows strong and gusty west tonorth-westerly winds of up to 50 km/h ahead of aneastward-moving cold front. In Sydney, themaximum temperature was 37.8°C and the rela-tive humidity dropped to 8 per cent by 3.00 pm.

7 JANUARY 1994

3.7NEW SOUTH WALES

BUSHFIRES, 1994


61

Synoptic chart for 7 January 1994

From the early afternoon a disastrous series ofevents unfold.• At 1.00 pm a bushfire breaks out in the Blue

Mountains area to the west of Sydney. This isa typical sandstone plateau area. The fireheads towards the town of Springwood andthreatens many homes.

• A fire at Mangrove Mountain, on a sandstoneplateau to the north of Sydney, forces theevacuation of several thousand homes in theGosford area. The fire is believed to be thework of an arsonist.

• A state of emergency is declared in the Cess-nock area as a fire, again thought to be the workof an arsonist, forces 200 residents to evacuate.

• Several serious fires, many the work of arsonists,threaten property and forests on the south coast.

• Firestorms sweep through several northernSydney sandstone plateau suburbs, destroying20 homes.

• Many other fires break out during the day andnight, several burning out of control.

The major impacts were economic, environ-mental and social.• Economic impacts: 205 homes and 20 other

buildings destroyed, $167 million cost• Environmental impacts: 800000 hectares burnt,

including sections of suburbs and nationalparks; thousands of native animals perished

• The main social impacts were: 4 deaths, 120people injured and 800 people left homeless.

1. Observe the diagram ‘Sandstone plateaus and the spread of bushfires’ and read the text on landforms.(a) Why is a fire more likely to start and spread on

the northward-facing slope?(b) How is the fire forced uphill?

IMPACTS OF THE FIRES

(c) Why do you think houses are built on the flat sandstone plateau? Why is this a very dangerous location in times of bushfires?

2. Observe the weather map for 7 January 1994 and read the information in the text.(a) Describe the weather situation over New South

Wales. (b) Why was the situation favourable for the

development of bushfires?3. Imagine you were able to observe an area of

sandstone plateau where the bushfire occurred. Describe what you might see.

Satellite image of Sydney and the central coast of New South Wales on 7 January 1994

Interpreting a satellite imageSatellite images allow large areas of the Earth to be shown in one image. In the satellite image above, the red areas are fires and the fine white line is the coastline. 1. Describe the location of the main fires. Is there

any pattern to their location?2. Where are the fires in relation to the main

populated areas? Locate each of the places mentioned on page 61.

3. Estimate the proportion of the map’s land area that shows fires burning.

Newcastle

Sydney

Wollongong

TOOLBOX

SOSE GEOG2 8.3.2

Darwin

Adelaide

Melbourne

Hobart

Brisbane

PerthSydneyCanberra

10

20

110 120 130 140 150160

17030

40

L

L

LM.S.L. ANALYSIS0000UTC 07 JAN 94

1008

1012

1012

1008

1004

1000

996992

10121008

10041000

996

992

1016

1016

1012

1008

1004 1000

1016

1020

984

988

980

1024

GEOactive 2

62

On 18 January 2003, Canberra was hit by the mostdevastating bushfire in its history. Canberra’sfirestorm had its beginnings on 8 January, whenlightning strikes in adjacent national parksignited a number of bushfires. Shifting windswidened the fire fronts of these fires, joining sometogether. Eight days later, strong south-westerlywinds drove the bushfire towards the nationalcapital.

The fierce winds blew burning embers ahead ofthe fire front, igniting spot fires in and aroundhouses on the capital’s south-western edge. Attimes, the shower of embers blew horizontally.There was little the 500 firefighters and 100police could do to stop such a massive blaze.Their work was made more difficult by the

HOW DID IT HAPPEN? billowing black smoke (which reduced visibility),by power and water-pressure failures, and by therapid spread of the fires.

The conditions in and around Canberra on 18 Jan-uary were perfect for a firestorm. There was ahuge supply of fuel. (Canberra is sometimes calledthe ‘bush capital’, as it is virtually surrounded bybushland and pine plantations and has a numberof bush/pine corridors.) This fuel was tinder drydue to the prolonged drought. On the day of thefire, very high temperatures, strong winds andextremely low relative humidity turned theadvancing bushfire into a fireball.

PERFECT FIRESTORM CONDITIONS

3.8BLACK SATURDAY,

CANBERRA 2003

ANATOMY OF AUSTRALIA’S MOST DEVASTATING ONE-DAY FIREThe difference between January 8 and January 18 was a few percentage points in humidity and slightly warmer temperatures plus ‘cyclonic’ winds.

� Humidity at 20%� Warm conditions of 30–35°C� Moderate intensity bushfire

started by lightning strikes

2–3 m high flames

TUESDAY, JANUARY 8:Brindabella — a moderate fire is started by lightning strikes in bushland on January 8.A normal bushfire fuelled by warm conditions, low humidity and drought conditions, with2–3 m flames spotting 100 m ahead. It is contained by firefighters.

Fires open up on three fronts north, west and south of Canberra. Fire crews are stretched and fire builds.

SATURDAY, JANUARY 18:Wind gusts of up to 100 km/h turn fire into an inferno. At this stage it becomes almost unstoppable.

Firefighters build containment lines to stop fire along a 35 km front. Bulldozers unable to enter rugged terrain.

100 km/h winds pick up on January 18 and change the fire’s behaviour.

Cumulus clouds of smoke up to 6000 m high, carrying embers and fuel

Embers

Crowning fires up to 60 m high

Fire front catches up to spot fires ahead and creates massive fireball.

Embers spotting 2 km ahead of main fire.

Grasslands and paddocks

� Humidity drops to 10%� Temperature rises to 48°C� Wind gusts of 60–100 km/h� Fire temperatures reach 1000°C� Crowning 60 m into the air 3 times

higher than the trees.


63

River

Murrum

bidgee

Path of the Canberra firestorm

The firestorm destroyed 530 homes — somein little more than 10 minutes — and about30 farms. Four people lost their lives, as didhundreds of cattle, thousands of sheep andmany thousands of native animals. About12 million trees were destroyed by the fire.Powerlines exploded and the famous MountStromlo Observatory was burnt down. Aswell as destroying lives, homes and liveli-hoods, the damage bill was expected to top$280 million.

1. What started the Black Saturday bushfires?2. From which direction was the wind blowing

when the firestorm reached Canberra’s urban edge?

3. Describe how spot fires played a role in the growth of this disaster.

4. Explain why conditions in Canberra on 18 January 2003 were perfect for a firestorm. Use examples and refer to the diagram.

5. Describe the spread of the fire from 8 January to 18 January. What action did the firefighters take?

6. List the main impacts of the bushfire.

7. Imagine that you are a news reporter for a Canberra television station. Write (and be prepared to deliver to the class) a three-minute news segment on the Canberra bushfire disaster.

8. Discuss as a class what actions you think should be taken by Canberra government bodies and private citizens to ensure a disaster such as Black Saturday never happens again.

9. To find out more about the Canberra bushfires of January 2003, go to www.jaconline. com.au/geoactive/geoactive2 and click on the Canberra Bushfire weblink for this chapter.

IMPACTS OF THE FIRE

Fire front races up Mt Stromlo and unleashes its force onto suburbs of Duffy, Chapman and Holder.

CANBERRA — Fire hits suburbs and housing and begins to run out of fuel but manages to destroy hundreds of homes. Firefighters’ efforts also manage to help stop its spread as conditions ease.

Trees uprooted and sparks sent flying into houses due to wind and fire.

Damage across the grassland was astounding according to scientists.

Grasslands and reserves

Pine plantation and observatory

Fire travels faster uphill, as it’s able to preheat the fuel in front of it from the hot air rising up the slope, bringing convection currents and burning embers rising on the hot air.

� Scientists call this ‘propagation’.

� This fire displayed an ‘impressive ability’ to feed itself.

Embers carried by high level winds start fires up to 12 km ahead of main fire.

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GEOactive 2

64

A storm is any violent disturbance of the atmos-phere and the effects associated with it. Themain types of storms are thunderstorms, tropicalcyclones, cold fronts and tornadoes (twisters). Inthis section, we look at thunderstorms and theireffect on people’s activities.

A thunderstorm is a storm associated with light-ning and thunder and occurs with cumulo-nimbus clouds. Cumulonimbus clouds can occursingly or extend over an area of 100 kilometresor more. Thunderstorms occur when cumulo-nimbus clouds build up enough static electricityto produce lightning. Lightning instantly heatsthe air through which it travels to about 20 000°Celsius — more than three times as hot as thesurface of the sun. This causes the air to expandso quickly that it causes an explosion (thunder).

WHAT IS A THUNDERSTORM?

The time between a lightning flash and thecrash of thunder tells you how far away thelightning is (5 seconds = 1.6 kilometres).

• Some 1000 years or so ago,the Vikings thought thunder was the rumblingof Thor’s chariot.(He was their god ofthunder and lightning.) Lightning marked the path of his mighty hammer, Mjöllnir, when hethrew it across the sky at his enemies.

• An average of about 100 severe thunderstorms are reported in Australia each year.

How a thunderstorm works

+ + ++

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+ + ++ +

+ ++

+

+

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+

–

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– –– –

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–

Lightning travels to the ground via the shortest route. This is why it sometimes strikes buildings or tall trees.

As air currents in a cumulonimbus cloud become more violent, they fling ice crystals and water droplets around faster. The more these crystals and droplets smash into one another, the more friction builds up. This creates huge energy stores of static electricity in the cloud.

Lighter particles with a positive electric charge drift upwards. Heavier particles with a negative charge sink.

The groundbelow the cloud has a positive charge.

The difference in energy between the positive charge on the ground and the massive negative charge at the bottom of the cloud becomes huge. A lightning bolt corrects some of this difference.

A bolt of lightning actually consists of a number of fltravel up and down between the cloud and the ground. This happens so quickly wcan’t see it.

Colddowndraft

Warm updraft

3.9NATURAL HAZARDS:

STORMS


65

Severe thunderstorms can be a serious naturalhazard because they can bring one or more of thefollowing features:• Hailstones, which are two centimetres or more

in diameter• Wind gusts of 90 kilometres or more per hour• Tornadoes, which are rapidly rotating columns

of air that descend in a funnel-shape fromthunderstorm clouds

• Very heavy rain, which can result in flashflooding. A thunderstorm often moves slowly,dropping a lot of precipitation in one area. Therain or hail may be too heavy and prolongedfor the ground to absorb the moisture. Thewater runs off, quickly flooding local areas.

1. Explain what is meant by (a) a storm,(b) a thunderstorm and (c) a severe thunderstorm.

2. Observe the diagram on page 64 and use information from the text to explain:(a) what causes static electricity in a cloud(b) what causes thunder and lightning(c) why a lightning bolt travels between the clouds

and the ground.3. Go to www.jaconline.com.au/geoactive/geoactive2

and click on the Storm weblink for this chapter.(a) Describe the ideal conditions for a severe

thunderstorm.(b) Outline the periods of the year when severe

thunderstorms are most likely to occur.

Interpreting a newspaper articleIn late 1998, storms in wheat-growing areas were an important contemporary geographical issue. Read the newspaper article on the right and answer the following questions:1. Describe the location of Boggabri.2. What three natural hazards had

affected Mr Jamieson’s crops in the previous two years?

3. How much money did Mr Jamieson lose because of the hailstorm?

4. Why do you think that Mr Jamieson was stoic (calm and resigned)?

5. How does heavy rain and hail affect a wheat crop?

SEVERE THUNDERSTORMS

TOOLBOX

Strike three: hail lashes farm after flood and

crop diseaseBy Andrew Clennell

Farmers around Gunnedah and Boggabri in north-westernNSW, reeling from recent flood damage and blighted crops,awoke yesterday to find hailstorms had shattered their hopesagain — a week before harvest time.

Mr Jock Jamieson, who lives 13 kilometres south-west ofBoggabri, said yesterday his entire wheat crop of more than200 acres — worth between $25 000 and $30 000 — had beenruined by hail.

The storms on Tuesdaynight follow the destruction ofmany crops by floods in July.

‘It’s bad news. The yearbefore we couldn’t get a cropin because it didn’t rain,’ MrJamieson said. ‘And now, thisyear, half of it was ruinedalready because of the floods.Now this has topped it off.’

Bitter harvest . . . Mr Jock Jamieson surveys his ruined crop,near Boggabri.

But Mr Jamieson was stoic.‘It’s part of farming, isn’t it,’ he said. ‘We have just got to

keep working.’The district agronomist for the Department of Agriculture

in Gunnedah, Mr Tony Dale, said there had been hail andheavy rain damage to crops south of Gunnedah betweenMullaley and Coonabarabran.

‘I don’t think it’s been that widespread, but it’s beenenough to do damage,’ he said.

Most of the crops were a week away from harvesting . . .Mr Xavier Martin, chairman of the NSW Farmers’ Associ-

ation grains committee, said the hail and rain may preventsome wheat and barley from being harvested for at least a weekbecause heavy machinery could not move into the paddocks.

‘The harvest was just getting under way and now this hap-pens,’ he said.

‘Heavy rain flattens the crop so the plant can’t be picked upby the headers. Hail knocks the grain right out of the head,leaving it to rot on the ground’. . .

Source: The Sydney Morning Herald, 19 November 1998

Narrabri Barraba

Manilla

Tamworth

Gunnedah

Boggabri

Hail damage

N

0 20 40 km


GEOactive 2

66

EdgecliffKings Cross

SydneyBroadwaySurry Hills

CamperdownPaddington

NewtownErskinevilleMarrickvilleSydenham

RedfernAlexandria

WaterlooArncliffe

Rockdale

MirandaCaringbahCronulla

N

790 to 1720

350 to 790

110 to 350

40 to 110

0 to 40

Number of claims

MosmanVauclusePotts PointPoint PiperRose BayDouble BayBellevue HillWoollahraBondiBondi JunctionBronteKensingtonRandwickCoogeeKingsfordRoseberyMaroubraBotanyMascotLa Perouse

0 3 6 km

At about 7.30 pm on 14 April 1999, a freak hail-storm hit Sydney. It affected more houses andpeople than any other natural disaster in Aus-tralia’s history. With a ferocity that shocked theunsuspecting residents and in the space of only45 minutes, more than 500 000 tonnes of hail-stones — some the size of cricket balls —destroyed homes and property. One person waskilled. More than 24 000 buildings were dam-aged. About 60 000 cars were damaged, with one-third so badly damaged they were written off bythe insurance companies. The total cost of thehailstorm was $2000 million.

In thunderstorms, when warm air rises abovefreezing level, water droplets can freeze and fallas hail. However, the hailstorm that hit Sydneyin April 1999 was caused by a much morepowerful thunderstorm — known as a super cell.The very strong updraft of air in a super-cellstorm keeps hailstones suspended inside thecloud for a much longer time than in regularstorms. The smaller hailstones join together andgrow before becoming too heavy and falling.

Development of a super-cell hailstorm

ANATOMY OF A HAILSTORM

Storm movement

Strong updraft of 10–15 km/hlifts moist air into freezing zone.

Hailstones leave theupdraft atspeeds of150 km/hand fall12 km toEarth.

Tropopause (top layer of atmosphere)

Strong updrafts keep hailstonessuspended much longer than regularstorms, allowing the hail to gain size.

Hailstones rise and fall, getting biggeras they gather moisture and refreeze.

Suburbs from which car insurance claims were lodged with NRMA Insurance by 20 April 1999

Stormtroopers cover SydneyArmy troops have been called in belatedly to help protect thehomes of Sydney’s distressed residents from further damageafter the disastrous hailstorm. They will join the StateEmergency Service and relief workers from the Rural FireService and New South Wales Fire Brigade, almost doublingthe number of workers to 5000 over the weekend.

The premier conceded that his government may not haveacted quickly enough to help storm victims deal withSydney’s biggest-ever storm disaster. The finger has alsobeen pointed at forecasters at the Bureau of Meteorology.By the time the direction and intensity of the storm cell wasidentified, it was already battering Sydney’s easternsuburbs. A new radar warning system designed to issuealerts to forecasters was installed at Kurnell one month ago,but meteorologists at the Weather Bureau had not been fullytrained to use it.

Many homes will need to be revisited in a month to havetarpaulins replaced as hundreds of tarpaulins ordered fromChina have been found to be below standard. Shortages ofterracotta tiles and skilled labour mean that many familiesface at least a six-month delay for permanent roof repairsand possibly a wait of several weeks for glass and carpetreplacement …

3.10THE SYDNEY

HAILSTORM, 1999


67

The dramatic aerial view of Kensington suggests the extent of the impact on the lives of residents. The patchwork of coloured tarpaulins over Sydney suburbs exceeded 90 000. More than 20 000 homes as well as schools and businesses suffered damage in an area concentrated on the eastern suburbs but stretching from Bundeena (south of the city centre) to the north shore.

1. Why was the Sydney hailstorm rated as Australia’s worst natural disaster? List the types of damage it caused.

2. Read the text and observe the diagram of the development of a super-cell storm. Write a paragraph explaining why the hailstones were larger than normal.

3. Observe the aerial photograph of Kensington.(a) How many roofs are not covered by tarpaulins?(b) What other features in the photograph are likely

to be hail-damaged?4. Read the report, ‘Stormtroopers cover Sydney’.

(a) Explain what is meant by the title of the article.(b) What change did the storm bring to the affected

communities of Sydney?(c) Which community groups were involved in the

disaster relief?(d) What problems were faced in repairing the

damage?(e) What lessons do you think organisations such as

insurance companies and community groups would have learnt from the Sydney hailstorm?

(f) Imagine you are premier of New South Wales. What action would you take to change the way disasters are handled?

Working with choropleth mapsThe choropleth map of car insurance claims resulting from the Sydney hailstorm gives a good picture of the path and severity of the storm. The map has been coloured to show the number of claims in each suburb. The darker shades show the highest number of claims and the lighter colours show the least number of claims. The colours have been carefully selected so that the observer can instantly see a pattern. Observe the map then complete the following paragraph to describe the number of insurance claims made in Sydney’s eastern suburbs.

More than 790 claims were made in a corridor of suburbs stretching from M in the south to P in the north. Between

and claims were made in the suburbs either side of this corridor, with the exception of W which had only 110 to 350 claims. Fewer claims were made in the coastal suburbs of L , C , and V , along with the southern suburb of M , the inner western suburb of A , and the north shore suburb of M .

TOOLBOX

GEOactive 2

68

The amount of energy released in a tropicalcyclone in one day is about the equivalent of theenergy released by 400 twenty-megatonnehydrogen bombs. If tropical cyclones occur nearlarge population centres, they can cause exten-sive damage to property and the loss of life.

A tropical cyclone may exist for only a few daysor as long as a few weeks. It can behave unpredict-ably, moving forward, hardly moving, changingdirection, and even doubling back on itself.

A moderate storm surge accompanied cyclone Aivu when it crossed the Queensland coast in 1989 causing severe damage to the village of Wunjunga.

Strong winds and high seas are among the effects of tropical cyclones.

Mean sea level 1 m normal high tide3 m surge

4 m storm tide

(b) Changes that occur during a storm surge

Mean sea level

Normal high tide

(a) Normal conditions before a storm

A tropical cyclone is a particular type of lowpressure system. It is called a hurricane in theUnited States and a typhoon in Asia. Tropicalcyclones are areas of warm, moist air risingrapidly. The upward flow of air is deflected by theCoriolis effect (winds deflected by the Earth’srotation), creating a rotation around a centralcore, known as the ‘eye’. Tropical cyclones areoften accompanied by very strong winds (gusts ofover 300 kilometres per hour have been recorded),torrential rain (1800 millimetres in 24 hours havebeen recorded) and very rough seas.

A storm surge can occur when a tropicalcyclone approaches or crosses a coastline. Thevery low atmospheric pressure and the stress ofstrong winds on the sea surface produce a rise insea level above the normal tide level (see the dia-gram). Among the destructive and death-dealingfeatures of tropical cyclones, sea action andfloods are ranked as more significant than winds.

Tropical cyclones need the energy provided bywarm water vapour (sea waters of at least 27°Celsius). Usually they die out if they move inlandaway from the water vapour, or out of the tropics,away from the warmth. This is why tropical coastalareas, such as the Caribbean Sea (CentralAmerica), the north-west Pacific and north-eastAustralia, between 5° and 15° north and south, arecommonly affected by tropical cyclones.

The formation of a tropical cyclone

Huge cumulonimbus clouds form.

Outflow

‘Eye’ of the cyclone

Warm air spirals up quickly.

Cool air flows into the central area to replace rising air.

Warm sea water is evaporated and rises up.

3.11NATURAL HAZARDS:

TROPICAL CYCLONES


69

The map shows the paths of major Australiancyclones since 1970. Tropical cyclones occur innorthern Australia between December and April,with the greatest activity usually occurringbetween January and March. The number ofcyclones varies considerably from year to year, thefrequency usually determined by the temperatureof the oceans in the tropical waters around northernAustralia. The Australian Bureau of Meteorologytracks an average of ten cyclones per year in theAustralian region. Of these, six may be expected tocross the Australian coast. We will examine twomajor tropical cyclones that have hit different partsof Australia and their effects on people.

Major Australian cyclones since 1970

On 20 December 1974, a low pressure systemseveral hundred kilometres north of Darwin wasnoticed by the Bureau of Meteorology. By late thenext day, satellite pictures indicated that it haddeveloped into a tropical cyclone. It was named‘Tracy’, and a warning was issued by the TropicalCyclone Centre in Darwin. Tracy intensified overthe next two days as it moved south-west andthen curved south-east towards Darwin. Thecyclone was tracked continuously by the Bureauof Meteorology at Darwin Airport.

Soon after midnight on Christmas Day, thecyclone approached the city with wind gusts inexcess of 100 kilometres per hour, and thedestruction of the city began. Tracy passeddirectly over Darwin. It had wind gusts ranging

from 217 to 240 kilometres per hour.Winds were abating in Darwin by 6.30 amas the cyclone weakened and moved fur-ther inland and degenerated into a raindepression.

Tracy was a small cyclone in area butvery intense, with a central pressure of950 hectopascals. Its destructive effect wastotal, because it passed directly overDarwin and hit just as the residents of thecity were preparing to enjoy themselves atChristmas.

Some of the destruction caused by Tropical Cyclone Tracy

TROPICAL CYCLONE TRACY, NORTHERN TERRITORY, DECEMBER 1974

Cyclone path

Canberra

Perth

Hobart

Melbourne

Alice Springs

Adelaide

Darwin

Tennant Creek

Sydney

Brisbane

6 to 10

Ada 1970

Althea 1971

Wanda 1974

Steve 2000

Tracy 1974Joan 1975

David 1976

Ted 1976Hal 1978

Alby 1978

Winifred 1986

Kathy 1984

Aivu 1989

Orson 1989

Nancy 1990

0

Average number of coastal crossings since 1959 (Zones indicate approximate frequency, not severity)

1 to 5 11 to 16

500 1000 km

Ian 1992Annette 1994

Chloe 1995

Barry 1996

Teresa 1992

Bobby 1995

Trixie 1975

Thelma 1998

Rona 1999

Vance 1999

Lena 1983Hazel 1979

N

Cairns

GEOactive 2

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1000

1004

996

996

992

1004

1008

1008

10041000

996 992

988

1012

1016

1016

1020

1020

1016

1012

1012

1016

[1010]

[1024]

[1006]

[970]

TROPICALCYCLONE “JUSTIN”

[1021]

10O8

1020

PT HEDLAND

ALICE SPRINGS

TOWNSVILLE

PT MORESBY

SYDNEY

BRISBANE

CANBERRA

ADELAIDE

MELBOURNE

HOBART

DARWIN

110120 130

140

150

10

20

30

40

50

160

170

PERTH

The size and structure of Tropical Cyclone Tracy and its path over Darwin on 25 December 1974

Impacts and responsesTracy was Australia’s most destructive cyclone. Itcaused the deaths of 65 people — of these, 16 were

reported missing at sea. As well, there were145 serious injuries and over 500 minorinjuries. Between 50 and 60 per cent ofbuildings were damaged beyond repair.Damage from the cyclone was over $4180million. Following the cyclone, more than35 000 people had to be evacuated — few ofthem still had homes, there were no essen-tial services and the threat of disease was amajor problem.

Many people reacted heroically, workingat great risk to rescue trapped people. Gov-ernment response included the provision ofemergency services and the building of anew Darwin — one that should be betterable to withstand any future tropicalcyclones. After Tropical Cyclone Tracy, newbuilding codes were introduced. Roofs fornew houses had to be tied to the foun-dations, and cladding was required to pro-tect homes from flying debris.

9.00 pm

Midnight3.00 am

4.00 amDarwin

Eye (light winds)Band of extremely high winds(average 140 km/h with gusts to 250 km/h)

Boundary of wind damage

Path of Tracy’s eye

Limit of the mostdestructive winds

N

0 50 km25

One of the largest tropical cyclones to threatenQueensland in more recent times was TropicalCyclone Justin, which formed off the coast ofnorth Queensland in March 1997. It broughtwinds of up to 150 kilometres per hour, largewaves, heavy rain and flooding. The cyclonestayed in the area for over two weeks with somechanges of position. It was unusual in that itremained almost stationary for up to five con-secutive days.

Tropical Cyclone Justin data

Synoptic chart for Australia on 9 March 1997

Date Time Latitude LongitudeCyclone category

Pressure (hPa)

Wind (km/h)

6 March 6 pm 17.0 S 153.5 E 0 9907 March 6 am 16.5 S 152.5 E 1 983 838 March 6 am 17.0 S 151.9 E 1 978 939 March 6 am 17.3 S 151.2 E 2 975 111

10 March 6 am 16.6 S 151.6 E 2 978 11111 March 6 am 16.6 S 151.9 E 2 982 9312 March 6 am 16.1 S 151.8 E 2 990 8313 March 6 am 14.9 S 152.5 E 1 985 7414 March 6 am 12.6 S 152.6 E 0 98015 March 6 am 11.3 S 154.6 E 1 980 8316 March 6 am 12.3 S 155.9 E 1 975 8317 March 6 am 12.5 S 155.5 E 3 960 12018 March 6 am 11.9 S 155.2 E 3 955 14819 March 6 am 12.5 S 153.6 E 3 970 14820 March 6 am 14.6 S 150.6 E 2 985 10221 March 6 am 15.6 S 147.3 E 1 995 8322 March 6 am 16.9 S 145.3 E 2 995 9323 March 6 am 18.1 S 145.5 E 0 1000

TROPICAL CYCLONE JUSTIN, QUEENSLAND, MARCH 1997

TROPICAL CYCLONEALERT NO. 4

Issued by the Darwin TropicalCyclone Warning Centre at 10.30 am

CST on 22/12/1974

Tropical Cyclone Tracy was centred180 km NW of Cape Don at 9.00 am and

moving WSW at 13 km per hour. Gales are not expected on the coast within 24 hours

but communities within Cape Don and Bathurst Island are alerted.

Next advice at 4 pmtoday.


71

Satellite image of Tropical Cyclone Justin, 9 March 1997

Impacts and responsesTropical Cyclone Justin left a path of destruction,including the devastation of sugar, pawpaw andbanana crops and severe damage to roads, bridgesand powerlines. Some buildings were also dam-aged — including the destruction of a $5 millionmarina in Cairns. Large areas of beach wereeroded by the huge waves and heavy swell. Therewas also some damage to parts of the Great BarrierReef. This is not unusual and not necessarilyserious, as the breakup of hard corals is seen bymany scientists as part of the natural ‘build-up andbreak-down’ cycle of reef and island development.

The total cost of the damage to Queensland was$150 million. Tragically, the cyclone also causedthe deaths of 33 people, 26 in Papua New Guineaand 7 in north Queensland.

1. What is a tropical cyclone?2. Why do tropical cyclones usually die out if they

move inland?3. Make a simple drawing showing the movement of

winds in and around a tropical cyclone.4. Why do you think tropical cyclones do not form

very close to the equator?5. Observe the map showing major Australian

cyclones since 1970 (page 69) and briefly describe the paths of the following three cyclones:(a) Annette (b) Hal (c) Althea.

6. Observe the map of Tropical Cyclone Tracy’s size, structure and path over Darwin.(a) What was the general direction of Tracy’s path

over Darwin?

(b) How wide was the belt of extremely high winds?(c) About how far did Tracy travel from midnight to

4.00 am?7. Prepare a multimedia presentation about Tropical

Cyclone Tracy. Search the Internet or go to www.jaconline.com.au/geoactive/geoactive2 and click on the Tropical Cyclone Tracy weblink for this chapter. Choose one of the following topics for your presentation:• Explain why Tropical Cyclone Tracy was so

destructive when it hit Darwin.• Describe the impacts of Tropical Cyclone Tracy.• Imagine you are a rescue worker and it is just on

dawn following the passage of Tropical Cyclone Tracy. Describe the tasks ahead of you. What would you do first?

• Choose your own topic.8. Describe how Australian governments responded

to Tropical Cyclone Tracy. To find out more, go to www.jaconline.com.au/geoactive/geoactive2 and click on the Cyclone Response weblink for this chapter.

9. What were the main destructive forces associated with Tropical Cyclone Justin? What were the main impacts?

10. Observe the synoptic chart and satellite image for9 March 1997. Describe the location of Tropical Cyclone Justin and the appearance of the cyclone from the satellite.

11. Using the information in the table of data, plot the movement of Tropical Cyclone Justin on a map of north Queensland using the following key: • category 0 • category 1• category 2 • category 3Include information such as dates, locations, direction, wind speeds and possible damage.



GEOactive 2

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Even though Australia is the driest of all theworld’s inhabited continents and has the lowestpercentage of rainfall as run-off, there areperiods of very heavy rainfall and floods. A floodis an unusual accumulation of water that over-flows from rivers, lakes or the ocean onto landthat is not normally covered by water.

There are three main types of flood:1. Slow-onset floods. These occur along the flood

plains of inland rivers, such as the Darlingand Namoi, and may last for weeks and evenmonths. They are caused by heavy rain andrun-off upstream, and the water can takedays and even weeks to affect farms andtowns downstream. The Great Floods of 1990are an example of a slow-onset flood (see theGeofacts).

2. Rapid-onset floods. These occur in mountainheadwater areas of larger inland rivers orrivers flowing to the coast. The rivers aresteeper and the water flows more rapidly.Rapid-onset floods are often more damagingbecause there is less time to prepare.

3. Flash floods. These occur due to heavy rainfallof short duration, such as in a severe thunder-storm. This type of flooding causes the greatestrisk of property damage and loss of life as itcan happen so quickly. It can be a seriousproblem in urban areas where drainagesystems are inadequate.

TYPES OF FLOODS

The Great Floods of 1990 in easternAustralia during April and May coveredmore than one million square kilometresof Queensland and New South Wales. Inboth states there was extensive damage totransport links and severe loss of stock.Many towns were invaded by floodwaters,and communities were isolated for longperiods.

Areas seriously affected by the Great Floods of 1990

On the flood plain, the stream erodes horizontally causing a river valley that is broad and flat. The river can no longer hold all the sediment it carries and deposits material to form flood plains and deltas.

Rivers begin in mountains. The force of water rushing down the steep slopes erodes vertically and carries the material away. Mountain river valleys are steep-sided and ‘V’-shaped.

A river is in the valley section when it reaches lower land downstream from the mountains. The valleys are deeper but wider as the river erodes both the floor and sides of the valley.

The life of a river

� �

�

3.12FLOODS AS

NATURAL HAZARDS

Nyngan

Charleville

Floods

Tropic of Capricorn

0 250 500 km

N


73

The town of Gympie in Queensland, during the record flood of February 1999. The commercial part of town was under 7 metres of water and the swollen Mary River peaked at 22 metres — its highest level since 1898. Six people died in the floods throughout south-east Queensland and the damage bill exceeded $20 million.

Floods are a natural occurrence, but they are anatural hazard to humans who have built farms,towns and transport routes in areas, such as floodplains, which are subject to flooding. A flood plainis an area of relatively flat land that borders ariver and which is covered by water during a flood.Flood plains are formed when the speed of thewater in the river slows down on flatter areas. Theriver begins to meander and gradually depositsalluvium. During this process, a flood plain andother landforms such as deltas are built up bythe river. These fertile, flat areas have becomefavoured for farming and settlement around theworld. In Australia many of our richest farmingareas are on flood plains. There are many towns inAustralia that have been built on flood plains closeto rivers. Parts of many towns are still located onflood plains and are subject to flooding.

FLOODS AND FLOOD PLAINS

The flat, fertile lands of the river flood plain are favoured areas for farming and settlement. The newer parts of towns are often built above the flood plain.

Floods are not confined to flood plains. If thereis sufficient volume of water, they can occur inthe lower parts of valleys and even on hill slopesduring periods of torrential rain.

A La Niña event in Australia is often associatedwith floods. La Niña translates from Spanish as‘the girl child’. A La Niña event is indicated by arise in the Southern Oscillation Index to well intothe positive (see the diagram of El Niño and theSOI, page 53). La Niña is virtually the opposite ofEl Niño. Very cold waters dominate the easternPacific, and the oceans off Australia are warmerthan normal. Large areas of low pressure extendover much of Australia; warm, moist air moves inand above average rainfall occurs. There can alsobe torrential rain and widespread floods. RecentLa Niñas in Australia have occurred in 1995–96when there was flooding in many parts of the con-tinent, particularly in western Queensland, and

in 1998 when many parts of Australia wereagain flooded, particularly north-

western New South Wales.

LA NIÑA AND FLOODS

Mountains Watershed

Delta

Waterfall

Tributary stream

Depositedsediment

Meander

Flood plain

GEOactive 2

74

The flooding of streams is caused by anumber of factors, the most importantbeing a high intensity of rainfall into acatchment or drainage basin. Highintensity rains may occur as a result of:• storms — thunderstorms, tropical

cyclones or rain depressions (the remains oftropical cyclones after they have moved inland)

• low pressure systems that cross Australiain the cooler months, including thoseassociated with cold fronts and eastcoast depressions.Cold fronts can bring heavy rain-

fall to southern Australia, particularlyduring winter and spring. East coastdepressions can bring heavy rain tothe coast of New South Wales inautumn and winter.

Cold fronts move from west to east across southern Australia, bringing cooler weather and rain.

East coast depressions form off the coast of New South Wales and can bring heavy rain and flooding.

WEATHER SYSTEMS AND FLOODS

1008 1008 1008

1012

1016

1012

992

996100010041008

10201016

1012

10001004

10081012

1016

1008

1012

H

L

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1012

1012

100810041000

1012 10161020

1024

992996

10001004

10081008

1012

H

H

H

L

L

L

The front of a cold air mass is called a cold front. Cold fronts are responsible for heavy rainfall and flooding in Australia in winter and spring.

The possibility of flood is also increased whenvegetation in catchment areas has been clearedor modified. Native vegetation can slow down therun-off and reduce the chance of flooding.

People could be discouraged from farming andliving on flood plains but this is not alwayspractical — especially as flood plains providesome of the best areas and soils for farming.Communities can, however, prepare for floodsand try to reduce their effects. This is calledflood mitigation. Flood mitigation programsinclude artificial levees (as built in Nyngan, seepage 80) to protect low-lying urban areas fromflooding, strengthening bridges, raising roadlevels and constructing large drains to movefloodwaters more rapidly.

FLOODS AND COMMUNITIES

Light showers may continue after the front has passed through.

Heavy rain, thunderstorms and strong winds may occur.

Lighter warm air rises.

Cumulonimbus clouds form.

Denser cold air pushes under the warm air.

Cold front


75

SAMPLE STUDYFlooding in north-east Victoria, October 1993The floods that hit north-east Victoria in October 1993were brought by rainfall that was a result of eastward-moving cold fronts. Record levels of rainfall poured intothe catchment areas of many of the rivers of north-eastVictoria, swelling 12 major rivers to flood levels. Muchof the area received over 150 millimetres of rainfall in24 hours. This resulted in the inundation of largeareas of flood plains and the houses and farms builton them. The Victorian Government declared a stateof emergency as more than 4000 residents of low-lying areas were evacuated. In the town of Benalla,2000 houses were inundated by floodwaters.

A flooded community in north-east Victoria, October 1993

1. Define a flood and describe the three main types of floods.

2. Explain why floods occur on flood plains and deltas.3. Describe the actions that can be taken by

communities to reduce the impacts of floods.4. What is La Niña? Why are floods often associated

with La Niña? To find out more, go to www.jaconline.com.au/geoactive/geoactive2 and click on the Flood weblink for this chapter.

5. Observe the diagram of the life of a river.(a) How are river valleys formed?(b) How are flood plains and deltas formed?

6. Read the sample study on the floods in Victoriain October 1993 and answer the following questions:(a) Observe the photograph of the flooded

community. Describe the scene.(b) Observe the satellite image of the cloud mass and

the weather map. What type of weather system is responsible for this cloud mass and heavy rainfall? Where is this weather system located on the weather map?

oods

in October 1993

Weather map for V


GEOactive 2

76

3.13NATURAL HAZARDS:

EARTHQUAKES

Earthquakes are shakings of the ground that aregenerated by disturbances in the Earth’s crust.

Scientists now believe that the Earth is made upof a series of layers.

The mantle, which makes up the bulk of theEarth’s interior, is semi-molten. This means thatit is a very slow-moving liquid. It circulates slowlybetween the very hot inner core (3700° Celsius)and the very cold outer crust (as cold as 0°). Thecrust on which we live is very thin and brittle.Over the last few billion years it has broken upinto a series of huge plates (called tectonic plates).These fit together like a giant jigsaw puzzle.Because the plates float on the mantle, they move.This movement is what scientists believe causesan earthquake.

The structure of the Earth is a bit like an apple: it has a core at the centre and a thin crust (skin) on the outside.

About 95 per cent of all earthquakes occurwhere two plates push together or rub past eachother. New Zealand, Japan and California sit onthe edge of two or more plates, while Australiasits much more safely in the middle of a plate.

Earthquakes are unpredictable and strikewithout warning. They range in strength fromslight tremors to severe shocks and can last fromjust a few seconds to as long as several days. Allearthquakes have a focus, which is the area under-ground where the Earth’s crust has snapped,sending shock waves to the surface. The point onthe Earth’s surface above the focus is called theepicentre. This is where the greatest amount ofdamage occurs. From the epicentre, shock waves

WHAT CAUSES EARTHQUAKES?

Outer core of molten metal

Inner core of solid metal

Convectioncurrent

Trench

Ridge

Mantle

Crust

radiate out like ripples in a pond (see the diagram).The shocks are less intense the further they arefrom the epicentre.

The focus and epicentre of an earthquake

Earthquakes are measured according to theirmagnitude and intensity. Magnitude (or energyreleased by an earthquake) is measured by theRichter scale. This scale is open-ended as thereis no upper limit to the amount of energy an earth-quake might release. The most severe earthquakesso far have not exceeded 9.5 on this scale. Anincrease of 1.0 on the scale indicates a 30 timesgreater magnitude. For example, the energyreleased at the magnitude of 6.0 is 30 timesgreater than the energy released at 5.0.

The intensity of an earthquake is measured onthe Modified Mercalli scale. It rates the amount ofdamage caused and uses Roman numerals at eachlevel (see the diagram). The intensity variesaccording to such factors as the nature of buildingsand the time of day. Intensity can vary for any givenearthquake whereas magnitude does not.

Since the start of the twentieth century,there have been only 18 earthquakes inAustralia measuring 6.0 or more on theRichter scale. This rate of occurrence ofless than one every five years contrastswith a world average of about 140 majorearthquakes per year.

Strongest shock

Weakest shock

Epicentre

Focus

MEASURING EARTHQUAKES


77

The Modified Mercalli scale

Not felt by people generally. Just recordable by seismograph.

Modified Mercalli scale

Reactionof people and buildings

A few people indoors notice a slight vibration.

Sleeping persons wake. Hanging items like lamps swing.

Things indoors fall over.

Old buildings suffer considerable damage — houses generally some damage.

Houses suffer damage. A few collapse.

Most houses damaged heavily or collapse.

Houses everywhere collapse. Complete destruction.

IX – XIIVII – VIIIVIV – VIVIVIIIII

An earthquake can cause massive damage.Buildings can be demolished; electricity and tele-phone lines cut; and gas, sewer and water mainscan be damaged. Landslides, subsidence and tsu-namis can also be triggered. Most injuries anddeaths result from falling objects and debris inand around buildings.

Earthquakes happen a lot in some places, andhardly at all in other places. We are fortunate inAustralia because, unlike many other countriesin the world, earthquake activity here is low. Infact, until the Newcastle earthquake in 1989,most people thought that earthquakes were not aserious natural hazard in Australia.

Australian earthquake risk areas

IMPACT OF EARTHQUAKES

AUSTRALIA’S RISK AREAS The San Francisco earthquake of 1906 killed over 3000 people and made more than half the population homeless, as 28 000 buildings were destroyed — mainly by fire.

Our most severe earthquakes have usuallyoccurred in unpopulated areas, but several havecaused damage in urban areas and others havecome very close. For example, an earthquakeoccurred near Sydney on 17 March 1999. Theepicentre was near the town of Wilton, in theSouthern Highlands between Sydney andGoulburn. The earthquake measured 4.7 on theRichter scale, not quite strong enough to causestructural damage. However, power was cut forover an hour, dams and pipelines were inspectedfor cracks, and coalmines were evacuated.

1. What are the focus and the epicentre of an earthquake?

2. How are earthquakes measured?3. What does the Richter scale measure?4. How much greater is the magnitude of an

earthquake of 8.0 than one of 7.0?5. Describe the damage caused by earthquakes with

the following measurements on a Modified Mercalli scale: (a) II (b) V (c) VI (d) IX–X.

6. Observe the map of Australian earthquake risk areas and the locations marked A, B, C and D on the map.(a) List these letters in terms of the greatest

earthquake risk (that is, the highest risk area first).(b) Locate where you live on the map. How great is

the risk where you live?

Perth

Hobart

Melbourne

Cairns

Alice Springs

Darwin

Tennant Creek

Sydney

Brisbane

120

0 500

Newcastle

Launceston

1000 km

60

40

Figures indicate peak ground velocity (mms-1).The higher the contour value, the greater the risk of earthquake within that area.

A

B C

D

Adelaide

Canberra

Tropic of Capricorn

N

GEOactive 2

78

SAMPLE STUDYNewcastle earthquake, 28 December 1989A killer earthquake occurred in Australia withoutwarning at 10.27 am on 28 December 1989,smashing into the city of Newcastle. Although itwas only a moderate earthquake, with a magnitudeof 5.6 on the Richter scale, the shock waves werefelt up to 500 kilometres away, across more thanone-quarter of New South Wales. In Sydney, 160kilometres to the south, there were reports of housewall cracks and computer screens trembling in cityskyscrapers.

Impacts of the earthquakeIt was the first earthquake sinceEuropean settlement in Australiato cause fatalities. There were13 deaths. Nine people werecrushed when a large sec-tion of the NewcastleWorkers Club collapsed,three were killed whenshopfronts collapsed inBeaumont Street, and oneperson died from shock.More than 160 people wereinjured. The loss of lifecould have been muchgreater if the earthquake hadstruck during school term andnot the Christmas holidays. Morethan 40 schools suffered structural damage, some sobadly that they had to be demolished. More than50 000 buildings were moderately to seriously dam-aged. The total cost of damage was more than fourbillion dollars, and the event caused the largestinsured loss due to disasters in Australian history.

There was unusually extensive damage for such arelatively small magnitude earthquake. This unu-sual damage was due to an underlying thin layer ofsilt and sand, covering a former course of theHunter River and a swamp. This appeared to mag-nify the shaking or ground motion.

Response to the earthquakeDue to the extent of the damage in such a lowseismic risk area, seismic activity is now closelymonitored. Since 1994, regulations have requiredthat all buildings must be constructed to resistearthquakes.

The earthquake caused irreparable damage to storefronts and other commercial buildings.

Sydney

Wollongong

Newcastle

Canberra

Gosford

Nowra

Tamworth

Lithgow

Port Macquarie

s

Muswellbrook

Batemans Bay

N

0 200 km100

1. Read the sample study on the Newcastle earthquake and answer the following questions.(a) How would you rate the Newcastle earthquake

on the Modified Mercalli scale? Give reasons for your answer.

(b) Why was there so much damage when the earthquake measured only 5.6 on the Richter scale?

(c) What were the main government responses to the Newcastle earthquake?

(d) Using information in the sample study and the photograph, describe the impact of the Newcastle earthquake.

2. Databases are available on many naturaldisasters, for you to access information. Imagine you are revisiting the earthquake area and wantto write your own brief story to describe it.To access information for your story, go to www.jaconline.com.au/geoactive/geoactive2 and click on the Newcastle Regional Library weblink for this chapter.

Worksheets3.3 Design a board game

The epicentre andseismic aftershocks



79

Use the rainfall graph below to answer the following questions.1. In which month does Nyngan normally experience

its highest rainfall? a. January c. Aprilb. February d. October

2. How much rain was recorded at Nyngan in April 1990?a. 30 millimetres c. 375 millimetresb. 60 millimetres d. 600 millimetres

Use the map showing rainfall depths in the Nyngan catchment to answer the following questions.3. The highest rainfall during April 1990 was

recorded to thea. east of Nyngan.b. west of Nyngan.c. south-west of Nyngan.d. south-east of Nyngan.

4. In April 1990 how much rainfall did the town of Tottenham receive?a. 350 millimetresb. Between 300 and 350 millimetresc. 300 millimetresd. Between 250 and 300 millimetres

Use the map of flood warning stations and table of peak discharges to answer the following questions.5. Where was the flood volume greatest?

a. Nynganb. Neurie Plainsc. Dandalood. Peak Hill

6. A discharge of 1610 m3/sec was recordeda. at Nyngan.b. 90 kilometres from Nyngan.c. 155 kilometres from Nyngan.d. 370 kilometres from Nyngan.

Broadsheet: Nyngan flood

0

50

100

150

200

250

300

350

400

Rai

nfal

l (m

m)

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Average rainfall1990 rainfall

Location of flood warning stations in the Nyngan catchment

Nyngan catchment rainfall depths, April 1990Graph of Nyngan rainfall

Peak discharges (cubic metres per second), April 1990

Peak Hill 344

Dandaloo 1610

Neurie Plains 678

Nyngan 2080

PangeeCreek

Creek

Creek

Creek

Bulbodney

Whitbarrow

Bullock

Burrill Ck

Brad

ys

CowalCowalMulia

BoganRiver

Darling R

Macquarie R

200

250

300

350

350

400

300

250Narromine

Peak Hill

Trangie

Nyngan

Tottenham

Nymagee

PangeeCreek

Creek

Creek

Creek

Bulbodney

Whitbarrow

Bullock

Burrill Ck

Brad

ys

CowalCowalMulia

Bogan River

0 20 40 km

N

250

Rainfall in millimetres

GEOactive 2

80

Use the aerial photograph and map of levee breaches to answer the following questions.7. A levee is a bank that

protects an area from flooding. Most floodwater flowed over or through levees to thea. north of Nyngan.b. south of Nyngan.c. east of Nyngan.d. west of Nyngan.

8. Which feature runs along the top of one levee?a. The railway lineb. Dandaloo Streetc. The airportd. The racecourse

Use the map of Nyngan on page 81 and other resources to answer the following questions.9. Describe the area subject to

flooding shown on the map.10. Suggest two reasons why

Nyngan received the worst flooding in the region.

11. Find two examples of transport routes cut by the April 1990 floods in Nyngan.

Aerial photograph of Nyngan in flood, 24 April 1990

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0 200 400 600 800 1000 m

Hospital

Wheatsilos

Racecourse

Showground

Electricity sub-station

Post Office

RailwayLevee breachesLow sections of leveeExisting leveeFlow path

Map of levee breaches in Nyngan


81

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Topographic map of Nyngan

textbook geoactive 2 chapter 3

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